Speed sensing method and apparatus

ABSTRACT

A method and apparatus for measuring the speed of a target object passing a pair of sensor units ( 12 ) displaced apart by a predetermined distant L in the direction of motion of the target object ( 16 ). Passage of one or more features of the target object ( 16 ) past the first sensor unit ( 12 A) results in the generation of a signal (x 1 ), and passage of the same feature of the target object ( 16 ) past the second sensor unit ( 12 B) results in the generate of a second signal, (x 2 ). A signal processor ( 18 ) is configured to determine a mathematical correlation between signals (x 1 ) and (x 2 ), and an associated time delay (τ 0 ). The speed (v) of the target object ( 16 ) is calculated by the signal processor ( 18 ) as the (ratio of the predetermined distance (L) to the time delay (τ 0 ).

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is related to, and claims priority from, U.S.Provisional Patent Application No. 60/417,839 filed on Oct. 11, 2002.

TECHNICAL FIELD

The present invention relates generally to speed sensors configured tomonitor the speed of a moving body such as a shaft or axle, and inparticular, to an improved speed sensing system utilizing a pair ofsensors each configured to detect random targets on a moving body, and asignal processor configured to measuring a phase shift between eachtarget detection signals, the phase shift proportional to a speed of thetarget.

BACKGROUND ART

Speed sensing plays an important role in monitoring, and thuscontrolling, machine operations. An accurate and reliable speed sensoris critical. Over the years numerous speed-sensing techniques anddevices have been developed. Mechanical speedometers, electro-mechanicalspeed sensors, magnetic speed sensors, and optical speed sensors arejust a few examples. Most popular speed sensing systems often include asingle sensor, an electronic control unit, and a target whose speedrelative to the single sensor is measured.

Depending upon the type of speed being measured, i.e., linear or angularspeed, and on the sensor technology that is employed, a target may beconstructed in a variety of ways and may take many different forms.Conventionally, speed sensing targets have been made from marked barsand toothed wheels, from multi-polar magnetic-strips and magnetic-rings,and from linear and angular bar-encoders. As the target moves relativeto the sensor, a conventional sensor output signal takes the form of aseries of pulses, with the pulse frequency being proportional to thetarget wheel speed.

The resolution or accuracy of these conventional speed sensing systemsdepend heavily, among other factors, on the accuracy of the spacingbetween the teeth in a toothed target, the spacing of the magnetic polesin a magnetic target, and the spacing of the bars in a bar encoder.Thus, for a precision system, a target with high spacing accuracy ispreferred.

However, the target manufacturing cost is proportional to the targetspacing accuracy requirements, and it is not always economical toconstruct a large outer diameter angular target wheel or a long lineartarget with high spacing accuracy. Accordingly, it would be advantageousto introduce a speed sensing system which maintains a high degree ofspeed measurement accuracy without requiring the production andapplication of a precision speed sensing target.

SUMMARY OF THE INVENTION

Briefly stated, the present invention sets forth a speed sensor systemcomprising a pair of sensing elements disposed in a directionally spacedrelationship adjacent a surface of a moving object from which a speedmeasurement will be acquired. A target, having substantially randomfeatures is disposed on or beneath the surface, and is moveddirectionally past the pair of sensing elements by the movement of theobject from which a speed measurement will be acquired. The pair ofsensing elements are directionally spaced apart by a predetermineddistance in the direction of the object's movement. Signals from each ofthe pair of sensing elements, generated by the passage of the target,are conveyed to a signal processor. The signal processor is configuredto determine a phase shift between the generated signals which isinversely proportional to the speed at which the target passed the pairof sensor.

As a method for measuring a target speed, the present invention includesthe steps of observing at a first point, a plurality of random featuresof said target, generating a first signal representative of saidobservations at said first point, observing at a second point displacedfrom said first point in a direction of motion of said target, saidplurality of random features of said target, generating a second signalrepresentative of said observations at said second point, andcalculating a phase shift between said first signal and said secondsignal, said phase shift inversely proportional to a speed of saidtarget.

The foregoing and other objects, features, and advantages of theinvention as well as presently preferred embodiments thereof will becomemore apparent from the reading of the following description inconnection with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings which form part of the specification:

FIG. 1A is a simplified diagrammatic view of one embodiment of a speedsensor system of the present invention in relation to a linearly movingobject;

FIG. 1B is a perspective view of the speed sensor system of FIG. 1A inrelation to a rotationally moving object;

FIG. 2A graphically illustrates sample eddy-current sensor signalsreceived from a pair of adjacent speed sensors units of the presentinvention;

FIG. 2B graphically represents a cross correlation function between thetwo signals shown in FIG. 2A;

FIG. 3A illustrates a first speed sensor configuration relative to thedirection of motion of a target object;

FIG. 3B illustrates a second speed sensor configuration relative to thedirection of motion of a target object;

FIG. 3C illustrates a third speed sensor configuration relative to thedirection of motion of a target object; and

FIG. 4 compares results in measuring angular speed from a speed sensorsystem of the present invention using a pair of eddy current sensorstogether with a toothless target wheel against the results from aconventional variable reluctance (VR) speed sensor system.

Corresponding reference numerals indicate corresponding parts throughoutthe several figures of the drawings.

BEST MODE FOR CARRYING OUT THE INVENTION

The following detailed description illustrates the invention by way ofexample and not by way of limitation. The description clearly enablesone skilled in the art to make and use the invention, describes severalembodiments, adaptations, variations, alternatives, and uses of theinvention, including what is presently believed to be the best mode ofcarrying out the invention.

Turning to FIGS. 1A and 1B, the basic components of a speed sensorsystem 10 of the present invention are shown. A pair of speed sensorunits 12A and 12B are disposed in a spaced relationship adjacent atoothless target surface 14 of a moving object 16, for example, thecircumferential surface of a bearing race as shown in FIG. 1B.Preferably, the speed sensor units 12A and 12B are spaced apart by apredetermined distance L between the speed sensor unit centers, alignedwith the direction of motion of the target surface 14, shown by thearrow in FIG. 1A. The sensor units 12A and 12B are operatively coupledto a signal processing unit 18. Preferably, as shown in FIG. 1A, thesensor units 12A and 12B are directly coupled to the signal processingunit 18 by electrically conductive wires 20 configured to communicaterespective signals x₁ and x₂ from the sensor units 12A and 12B to thesignal processing unit 18. However, those of ordinary skill in the artwill recognize that a variety of components may be utilized to couplethe sensor units 12A and 12B to the signal processing unit 18, includingwireless transmission components.

The operating principles of the current invention are based ongenerating and analyzing two mathematically correlated signals x₁ and x₂from the pair of sensor units 12A and 12B in the speed sensor system 10.By detecting the phase shift between corresponding points in each of thetwo signals x₁ and x₂, a time delay can be determined. The phasevariation in signals is solely related to the speed of motion. The speedof motion can be calculated as the ratio of the distance L between thesensor units 12A and 12B, and the determined time delay.

For accuracy considerations, it is highly desirable to use randomsignals, each preferably having a high frequency content. To this end, atarget surface 14 with random or near random topographical features(roughness) is employed as target to produce randomly variable signals.With certain types of sensor units 12A and 12B, such as eddy currentsensors, the randomness of the signals x₁ and x₂ can be enriched bysubsurface material property variations.

The first sensing unit 12A and the second sensing unit 12B areconfigured to be substantially sensitive to surface and/or subsurfacefeatures of the target surface 14, and are functionally similar in thateach sensor unit 12A, 12B produces identical signals or substantiallysimilar signals when passing over the same surface or subsurfacefeatures on the target surface 14. Alternative sensor units 12 mayinclude optical sensors sensitive to optical variations on the targetsurface 14.

As the target surface 14 moves relative to the speed sensor system 10,the first sensing unit 12A and the second sensing unit 12B each generatesignals, such as exemplified in FIG. 2A, at an identical sampling rate fthat is substantially higher than the signal variation rate, allowingthe speed sensing system 10 to resolve high frequency surface featureseven at the highest target speeds.

In general, a correlation exists between the first signal x₁=[x₁₁, x₁₂,x₁₃, . . . x_(1j), . . . , x_(1n)] generated by the first sensing unit12A and the second signal x₂=[x₂₁, x₂₂, x₂₃, . . . x_(2j), . . . ,x_(2n)] generated by the second sensing unit 12B in response to thepassage of surface or subsurface features on the target surface 14,where n represents the sample size (number of data points in a sample).There is, however, a time delay of $\tau_{0} = \frac{m}{f}$between the first signal x₁ and second signal x₂ where m represents thenumber of shifted data points. The direction of the signal shiftingcorresponds the direction of the motion of the target surface 14relative to the sensor units 12A and 12B.

Thus, a cross correlation function y(τ) between the signals x₁ and x₂may be defined by the equation:y(τ)=∫x ₁(t+τ)·x ₂(t)dt  Equation (1)

-   -   which reaches a maximum value when τ=τ₀.

The time delay τ₀ can be determined by finding the maximum value of thecross correlation function of the signals x₁ and x₂. That is:τ₀=ψ(y _(max))  Equation (2)

-   -   where ψ is an inverse function to the cross correlation function        y(τ) defined in Equation (1).

During operation, signal processor 18 receives and processes signals x₁and x₂. The incoming signals x₁ and x₂ are initially processed to removeany direct current (DC) components, resulting in a pair of signals eachhaving zero-mean such as shown in FIG. 2A. The signal processor 18further performs the cross correlation analysis of the two signals,preferably using a Fast Fourier Transform (FFT) based algorithm for fastcomputation. Next, the signal processor 18 determines the time delay τ₀between the two signals by calculating the maximum value for the crosscorrelation function y(τ) defined in Equation (1). Finally, the speed ofmotion v for the target surface 14 past the sensor units 12A and 12B iscomputed by the signal processor 18 as: $\begin{matrix}{v = \frac{L}{\tau_{0}}} & {{Equation}\quad(3)}\end{matrix}$

Optionally, the signal processor 18 may be configured to compute arelative position of the target surface 14 by integrating the computedspeed v with respect to time.

Returning to FIG. 2A, the signals x₁ and x₂ illustrated graphically arerepresentative of signals from a pair of independent eddy current speedsensor units 12A and 12B positioned 0.788 inches apart along thecircumferential direction of motion for a rotating target surface 14,such as shown in FIG. 1B. For a sampling rate of approximately 48 kHz,the resulting sample size is 3700 data points for a single revolution ofthe target surface 14. In the graph shown in FIG. 2A, the horizontalaxis represents sample sequencing and vertical axis is the strength ofthe signals in volts. The cross correlation function of signals x₁ andx₂, illustrated in FIG. 2B, identifies a maximum value at data point4036 that corresponds to a shifting of 336 data points (4036-3700=336).The corresponding time delay between the first signal x₁ and secondsignal x₂ is τ₀=336/48000=0.007 sec. The surface speed of the target 14is then v=0.788/0.007=112.6 in/sec. The direction of motion isdetermined by the direction of signal shifting.

Based on the selected sensor technology, the signal processor 18 andsensor units 12A, 12B could be integrated into a single unit 20, such asshown in FIG. 1B using modern ASIC fabrication techniques with DigitalSignal Processing (DSP) computation ability.

To ensure a good correlation between the two signals x₁ and x₂ underless than ideal installation and/or application conditions, differentialsensing combinations of speed sensor units 12 may optionally be used. Inthis case one sensor combination may contain more than two speed sensingunits 12. A comparison of signals from each of the speed sensor units 12comprising the differential sensing combinations permits removal orcancellation of signal components common to all sensing units 12, suchas noise or interference, which are present at each speed sensor unitlocation. These common signal components usually carry no informationwith respect to signal phase shifting.

FIGS. 3A through 3C illustrate three differential sensing combinationsand the positioning of the associated speed sensing units 12 inrelationship to the direction of motion of the target surface 14. InFIG. 3A, a first sensor combination 100 contains four speed sensor units12A-12 D positioned at the corners of a rectangle to form twodifferential sensing pairs. The first differential sensing pair isformed by speed sensor units 12A and 12C, and the second differentialsensing pair is formed by speed sensor units 12B and 12D. In the firstdifferential sensing pair, speed sensor units 12A and 12C are separatedby a center distance W perpendicular to the direction of motion. In thesecond differential sensing pair, speed sensor units 12B and 12D aresimilarly separated by the center distance W, perpendicular to thedirection of motion. Each differential sensing pair is spaced apart by adistance L substantially in the direction of motion.

FIG. 3B illustrates an alternate arrangement for a sensing system 200wherein the differential sensing pairs 12A, 12C and 12B, 12D aredisposed at the corners of a parallelogram, i.e., where the center linesbetween speed sensor units 12A and 12C and between speed sensor units12B and 12D are not perpendicular to the center line defined by theposition of speed sensor units 12A and 12B. The included angle α betweenspeed sensor units 12A, 12C and speed sensor units 12A, 12B is set equalto the included angle β between speed sensor units 12B, 12D and speedsensor units 12A, 12B. That is α=β, such that the placement of the speedsensor units 12A-12D defines a parallelogram having two sides parallelto the direction of motion of the target surface 14. In general, α and βcould each vary from 0 to 360 degrees.

FIG. 3C shows an alternate arrangement for a sensing system 300 similarto that shown in FIG. 3A, but where the first pair of differentialsensing units 12 A, 12C and the second pair of differential sensingunits 12B, 12D are disposed in two different sensor housings, and henceare spaced apart by a distance L′>L. As is shown in FIG. 3A, thecenterline between the centers of sensing elements 12A and 12Bsubstantially aligns with the direction of motion of the target surface14. Correspondingly, the center line that connects the centers of thesensing elements 12C and 12D is also substantially aligned with thedirection of motion. The center lines connecting the centers of thefirst pair of differential sensing units 12A, 12C is parallel with thecenter line that connects the centers of the second pair of differentialsensing units 12B, 12D, and substantially perpendicular to the directionof motion for the target surface 14.

The current invention is not confined to any specific type of speedsensors units. However, the speed sensor units 12 are preferably eddycurrent sensors capable of generating signal variations induced both bytopographical features on the target surface 14 and by subsurfacematerial property changes in the target object 16. This allows thesensing system to be used not only for rough target surfaces 14 but alsofor smooth target surfaces 14 where the signal variation is inducedprimarily by subsurface material property changes rather than by surfacetopographical features.

FIG. 4 graphically illustrates the validity of the sensing system 10 andtechniques of the present invention in measuring angular speed using apair of eddy current speed sensor units 12A, 12B and a toothless targetobject 16. The graph of FIG. 4 plots the angular speed of the targetobject 16 as measured by the sensing system 10 of the present inventionversus the angular speed of the target object 16 as measured by aconventional variable reluctance (VR) speed sensor system, illustratinga close correlation between the two sensor systems.

It should be understood that the sensing system 10 and techniques of thepresent invention are applicable to a host of applications such as foruse in bearing application, and particularly in bearing applicationswherein the target surface 14 is a bearing seal.

In view of the above, it will be seen that the several objects of theinvention are achieved and other advantageous results are obtained. Asvarious changes could be made in the above constructions withoutdeparting from the scope of the invention, it is intended that allmatter contained in the above description or shown in the accompanyingdrawings shall be interpreted as illustrative and not in a limitingsense.

1. A speed sensing system for measuring the speed of a target object,comprising: a first differential speed sensor unit operatively disposedadjacent a surface of said target object, said first differential speedsensor unit configured to generate a first differential signalresponsive to the passage of at least one random feature of said targetobject; a second differential speed sensor unit operatively disposedadjacent a surface of said target object and displaced at apredetermined distance from said first differential speed sensor unitsubstantially in a direction of motion of the target object, said seconddifferential speed sensor unit configured to generate a seconddifferential signal responsive to the passage of said at least onerandom feature of said target object; and a signal processor configuredto receive said first and second differential signals, said signalprocessor further configured to apply a cross correlation analysis todetermine a phase shift between said first and second differentialsignals, said phase shift inversely proportional to a speed of saidtarget object.
 2. (canceled)
 3. (canceled)
 4. (canceled)
 5. The speedsensing system of claim 1 wherein said first and second differentialspeed sensor units each include at least one eddy current sensor; andwherein said at least one feature is a random subsurface target feature.6. The speed sensing system of claim 1 wherein said first and seconddifferential speed sensor units each include at least one opticalsensor.
 7. The speed sensing system of claim 1 wherein said signalprocessor is configured to filter direct-current components from saidfirst and second generated differential signals such that said generateddifferential signals have a zero signal mean.
 8. The speed sensingsystem of claim 1 wherein said signal processor is configured utilize aFast Fourier Transform-based algorithm to determine a cross correlationfunction between said generated differential signals, said crosscorrelation function defined by:y(τ)=∫x ₁(t+τ)·x ₂(t)dt where x₁ is said first generated differentialsignal; x₂ is said second generated differential signal; t is a signaltime; and τ is a time delay between said generated differential signals.9. The speed sensing system of claim 8 wherein said phase shift isassociated with a maximum value for said cross correlation function; andwherein said signal processor is further configured to determine amaximum value for said cross correlation function; wherein a speed v ofsaid target object is determined from: $v = \frac{L}{\tau_{0}}$ where Lis said predetermined distance; and τ₁ is a time delay corresponding tosaid determined maximum value for said cross correlation function. 10.The speed sensing system of claim 1 wherein said first differentialspeed sensor unit and said second differential speed sensor unit aredisposed within a common housing.
 11. The speed sensing system of claim1 wherein said at least one random target feature is a surface featureof the target object.
 12. The speed sensing system of claim 1 whereinsaid at least one random target feature is a subsurface feature of thetarget object.
 13. The speed sensing system of claim 1 where each ofsaid first and second differential speed sensing units has an identicalsampling rate; and wherein said identical sampling rate is substantiallygreater than a signal variation rate for said first and seconddifferential speed sensing units.
 14. A method for speed measurement ofa target object, comprising the steps of: observing at a first point, apassage of at least one random feature of the target object; generatinga first signal responsive to said passage of said at least one randomfeature at said first point; observing at a second point, displaced at apredetermined distance from said first point in a direction of motion ofsaid target object, said passage of said at least one random feature ofthe target object; generating a second signal responsive to said passageof said at least one random feature at said second point; observing at athird point, displaced from said first point at least perpendicularly tothe motion of said target object, a passage of at least one additionalrandom feature of the target object: generating a third signalresponsive to said passage of said at least one additional randomfeature of the target object; observing at a fourth point, displaced atsaid predetermined distance from said third point in a direction ofmotion of said target object said passage of said at least oneadditional random feature of the target object; generating a fourthsignal responsive to said passage of said at least one additional randomfeature at said second point; determining a first differential signalfrom said first and third generated signals; determining a seconddifferential signal from said second and fourth generated signals;filtering direct-current components from said first and second generatesdifferential signals; applying a cross correlation analysis with a FastFourier Transform-based algorithm to calculate a phase shift betweensaid filtered first and second differential signals, said phase shiftinversely proportional to a speed of said target object.
 15. The methodof claim 14 for speed measurement of an object wherein said phase shiftis associated with a maximum value of a cross correlation functionbetween said filtered first and second differential signals, and whereinsaid step of applying further includes calculating said maximum value ofsaid cross correlation function between said filtered first and seconddifferential signals, said cross correlation function defined by:y(τ)=∫x ₁(t+τ)·x ₂(t)dt where x₁ is said first differential signal; x₂is said second generated differential signal; t is a signal time; and τis a time delay between said first and second differential signals. 16.The method of claim 15 for speed measurement of an object, furtherincluding the step of determining a speed v of said target object from:$v = \frac{L}{\tau_{0}}$ where L is said predetermined distance; and τ₀is a time delay corresponding to said determined maximum value for saidcross correlation function.
 17. (canceled)
 18. A method of claim 14 forspeed measurement of a target object further including the step of:determining a relative position of the target object from saidcalculated phase shift.
 19. The method of claim 18 for determining arelative position of a target object wherein said determining stepincludes the step of integrating a calculated speed of said the targetobject with respect to time.
 20. The speed sensing system of claim 1wherein said first differential speed sensor unit includes first andsecond speed sensors spaced at least perpendicular to a direction ofmotion of the target object, each of said first and second speed sensorsconfigured to generate a signal responsive to the passage of at leastone feature of said target; wherein said second differential speedsensor unit includes third and fourth speed sensors spaced at leastperpendicular to a direction of motion of the target object, each ofsaid third and fourth speed sensor units configured to generate a signalresponsive to the passage of at least one feature of said target object;wherein said first and third speed sensors are disposed along a commonline parallel to the direction of motion of the target object; whereinsaid second and fourth speed sensors are disposed on a second commonline parallel to the motion of the target object; wherein said firstdifferential signal is representative of a difference between saidsignals generated by said first and second speed sensors; and whereinsaid second differential signal is representative of a differencebetween said signals generated by said third and fourth speed sensors.21. The speed sensing system of claim 20 wherein said signal processoris configured to cancel signal components common to said signalsgenerated by said first, second, third, and fourth speed sensors. 22.The speed sensing system of claim 20 wherein said first and third speedsensors are configured to observe surface features of said targetobject; and wherein said second and fourth speed sensors are configuredto observe subsurface features of said target object.
 23. The speedsensing system of claim 20 wherein said first, second, third, and fourthspeed sensors define a parallelogram having two sides parallel to thedirection of motion of said target object.
 24. The speed sensing systemof claim 23 wherein said first, second, third, and fourth speed sensorsdefine a rectangle having two sides perpendicular to the direction ofmotion of said target object.